The Lost Era-series of books is set inbetween the Enterprise-B part of GEN and TNG and deals with momentous events during that time-period of 70 years, featuring some of the less known characters.
"The Sundered" is set in 2298 and focuses on the Excelsior under the command of Captain Sulu, with Chekov and Tuvok and others starring as well. The Excelsior is sent on a diplomatic mission to negotiate a peace treaty with the Tholians who are secretly engaged in a war with a species called the Neyel who, as is later discovered, are the offspring of a human colony that got flung deep into space during an experiment on an early version of Cochrane's warp engine.
Very good characterization, interesting dilemma concerning the Prime Directive, especially the question how far (Earth's) humanity is responsible for the actions of a branch of humans that developed under far worse conditions and had no contact with Earth for 200 years... a definite recommendation here.
remarks originally written in 2006